In a Long Term Evolution (LTE) system or a Long Term Evolution Advanced (LTE-A) system, an uplink symbol is referred to as a single carrier frequency division multiple access (SC-FDMA) symbol, and a downlink symbol is referred to as an orthogonal frequency division multiplexing (OFDM) symbol. If an uplink multiple access mode of orthogonal frequency division multiple access (OFDMA) is introduced into a subsequent technology, the uplink symbol may also be referred to as an OFDM symbol. The uplink symbol and the downlink symbol are collectively referred to as a symbol.
From a perspective of a time dimension, a time length of one radio frame is 10 milliseconds (ms), a time length of one subframe is 1 ms, and one radio frame includes 10 subframes. There are two subframe formats. One format is a normal cyclic prefix (NCP) subframe format. One NCP subframe includes 14 symbols or two slots (a time length of one slot is 0.5 ms). The symbols are numbered from 0 to 13, symbol 0 to symbol 6 are even-numbered slots, and symbol 7 to symbol 13 are odd-numbered slots. The other subframe format is an extended cyclic prefix (ECP) subframe format. One ECP subframe includes 12 symbols or two slots. The symbols are numbered from 0 to 11, symbol 0 to symbol 5 are even-numbered slots, and symbol 6 to symbol 11 are odd-numbered slots. From a perspective of a frequency dimension, a smallest unit is a subcarrier. A subcarrier spacing in the LTE system is 15 kHz. A subcarrier spacing in a 5G communications system may change, and may be a multiple of 15 kHz, for example, 15 kHz, 30 kHz, or 60 kHz.
From a perspective of both the time dimension and the frequency dimension, a smallest unit of a resource used for communication antenna port transmission is a resource element (RE). One RE includes one symbol in time domain and one subcarrier in frequency domain. A resource element group (REG) may include an integral quantity of REs. For example, one REG may include 4 or 16 REs. One physical resource block (PRB) includes one slot in time domain and 12 subcarriers in frequency domain. One subframe includes one PRB pair. One resource block (RB) includes one subframe in time domain and 12 subcarriers in frequency domain. A resource block group (RBG) may include an integral quantity of PRBs. For example, one RBG may include one, two, three, or four PRBs, or another integral quantity of PRBs.
In the LTE system, a physical channel is used to transmit data information and/or control information. The physical channel includes one or a combination of the following: a physical uplink shared channel (PUSCH), a physical uplink control channel (PUCCH), a physical downlink control channel (PDCCH), an enhanced physical downlink control channel (EPDCCH), a physical control format indicator channel (PCFICH), a physical hybrid automatic repeat request indicator channel (PHICH), a physical downlink shared channel (PDSCH), and the like, or is a channel that is newly introduced in an existing communications standard and that has a same function but has a different name, such as a control channel or a data channel introduced in short transmission time interval (TTI) transmission. PDSCHs/PUSCHs in LTE all are shared channels, and a TTI is defined as 1 ms, to be specific, a length of one subframe. A base station performs one time of user scheduling in each TTI (or each 1 ms), and scheduled user data needs to be sent in each TTI. In addition to being defined as 1 ms, the TTI may be of another length. For example, a length of a TTI in the 5G communications system may change, and the length of the TTI may be 125 μs, 250 μs, 500 μs, 750 μs, or 1 ms.
A pilot signal in the LTE system includes a common demodulation reference signal (CRS), a downlink demodulation reference signal (DL DMRS), an uplink demodulation reference signal (UL DMRS), a channel state information-reference signal (CSI-RS), a positioning pilot signal (PRS), a primary synchronization signal (PSS), a secondary synchronization signal (SSS), a discovery pilot signal (DRS), and a sounding reference signal (SRS). The CRS is used for downlink channel estimation, and is used for coherent detection and demodulation by user equipment. The DL DMRS and the UL DMRS are used for data demodulation. Both the CRS and the DL DMRS are downlink reference signals. The UL DMRS is an uplink reference signal. A difference between the CRS and the DL DMRS lies in that the CRS does not include precoding information, but precoding may be performed on the DL DMRS. The CSI-RS is mainly used for downlink channel measurement, and the SRS is mainly used for uplink channel measurement. The PRS is mainly used for positioning measurement. The PSS is mainly used to help the user equipment implement symbol timing and frequency synchronization, and is combined with the SSS to help the user equipment identify a cell identity (ID). The DRS is a pilot channel similar to a synchronization channel in the LTE system, and may carry information such as the cell ID, but time domain density of the DRS is significantly lower than that of the synchronization channel, and a time domain interval may reach a magnitude of seconds. After a small cell is disabled, the small cell still periodically sends a DRS. Because of sparsity of the DRS, the DRS has little impact on power consumption and interference. The user equipment always searches for a DRS during movement. If the DRS is detected, it indicates that the user equipment has entered coverage of a small cell, and the user equipment may further enable a mechanism such as uplink paging to wake up the small cell to serve the user equipment.
When data is transmitted between the base station and the user equipment, in the TTI, the UL DMRS and the DL DMRS are used as dedicated demodulation pilot signals. The CRS is used as a common demodulation pilot signal. The CSI-RS and the SRS are used as measurement pilot signals. Additionally, the PSS, the PRS, the SSS, and the DRS are used as positioning and/or synchronization pilot signals. Each PRB of each TTI includes a CRS, and the CRS exists in full bandwidth.
FIG. 1 is a pattern of a pilot signal that is in a TTI and that is sent by a base station to user equipment in a current system. Only a CRS in FIG. 1 is analyzed to obtain a pattern on the left side of FIG. 2. A PRB with a normal cyclic prefix (Normal CP) includes seven symbols, and a PRB with an extended cyclic prefix (Extended CP) includes six symbols. A pilot symbol carries a pilot signal. If different transmit antenna ports of a same base station separately transmit pilot signals and data at the same time, receiving the pilot signals by the user equipment is interfered with. To avoid such a case, when a transmit antenna port transmits a pilot signal, other transmit antenna ports of the same base station vacate corresponding time-frequency resources. For example, in FIG. 1, when an antenna 0 transmits a pilot signal (to be specific, a pilot symbol R0 of the antenna 0 carries the pilot signal), an antenna 1, an antenna 2, and an antenna 3 do not transmit pilot signals or data (to be specific, a pilot symbol R1 of the antenna 1, a pilot symbol R2 of the antenna 2, and a pilot symbol R3 of the antenna 3 do not carry the pilot signals or the data). In addition, an absolute location of a pilot subcarrier in frequency domain is related to a cell ID, that is, a pilot subcarrier in which a pilot symbol is located in FIG. 2 does not change in a same cell.
Only the CRS is analyzed above. When there is data to be transmitted in the TTI, the dedicated demodulation pilot signal necessarily exists in the TTI. It can be learned from FIG. 1 that, when there is data to be transmitted in the TTI, a physical resource block of each resource block includes a common demodulation pilot signal and a dedicated demodulation pilot signal. In addition, the TTI further includes another type of pilot signal. Consequently, each resource block of the TTI is filled with pilot symbols, and the foregoing interference problem of the pilot signal needs to be considered. If a pilot port of the base station is extended, a new pilot port needs to occupy a non-pilot symbol as a pilot symbol, and therefore it is difficult to extend a new pilot port in a communications system.